This invention relates to cooperative identification systems (which had their electronic beginnings in World War II as Identification xe2x80x94Friend or Foe Systems) in which the identifying agency and the object to be identified cooperate in the identification process according to a prearranged scheme. More specifically, the invention relates to systems consisting generically of an interrogator (or xe2x80x9creaderxe2x80x9d) inductively coupled to a transponder (or xe2x80x9ctagxe2x80x9d) where the reader is associated with the identifying agency and the tag is associated with the object to be identified.
Such systems are being used or have the potential of being used for identifying fish, birds, animals, or inanimate objects such as credit cards. Some of the more interesting applications involve objects of small size which means that the transponder must be minute. In many cases it is desirable to permanently attach the tag to the object which means implantation of the device in the tissues of living things and somewhere beneath the surfaces of inanimate objects. In most cases, implantation of the tag within the object forecloses the use of conventional power sources for powering the tag. Sunlight will usually not penetrate the surface of the object. Chemical sources such as batteries wear out and cannot easily be replaced. Radioactive sources might present unacceptable risks to the object subject to identification. One approach to powering the tag that has been successfully practiced for many years is to supply the tag with power from the reader by means of an alternating magnetic field generated by the reader. This approach results in a small, highly-reliable tag of indefinite life and is currently the approach of choice.
For many applications, convenience and utility dictate that the reader be hand-portable which translates into the use of batteries to power the unit. However, the size and weight of batteries having the requisite capacity to perform the identification function at reasonable ranges without interruption challenge the very concept of hand-portability. The twin goals of ease of use and system performance have been the subject of uneasy compromise in the past. There is a need to harness the recent advances in technology to the design of energy efficient systems in order to realize the full potential of identification systems based on inductive coupling.
As identification systems of this type proliferate and users multiply, it becomes important to recognize this changing environment in the design of next-generation identification apparatus. Newer-model readers should be able to read older-model tags. Users"" privacy and security interests must be respected one user should not be able to read another user""s tags. And finally, in this computer-driven world, it must be possible to conveniently interface readers with computers.
The electronic identification system with improved sensitivity provides two-way communication between reader and tag by a one-step or two-step modulation process in which the information to be communicated either modulates an alternating magnetic field directly or modulates a periodic signal which modulates an alternating magnetic field.
Generally, in order to obtain the highest possible communication sensitivity, the coil and capacitor in both reader and tag are maintained at or near a state of resonance while communications are taking place by adjusting either intermittently or continually the frequency of the coil driving signal, the inductance of the coil, or the capacitance of the capacitor in the reader and the inductance of the coil or the capacitance of the capacitor in the tag. It may be desirable in certain situations, in order to realize the best communication performance, to maintain the coil and capacitor near resonance but not in a state of resonance.
In order to maximize the alternating magnetic field produced by the reader coil, the driving signal is tailored to the characteristics of the resonant circuit so that the highest possible coil current is achieved. In this regard, the coil is driven push-pull by means of high-power field-effect transistors connected in a bridge arrangement. Highly effective impedance matching is achieved by transformer coupling of the coil and the driver and capacitors.
Transformer coupling of the tag coil to the other devices and circuits in the tag is used in order to satisfy the diverse matching requirements imposed by these other devices and circuits.
The system utilizes maximum-likelihood procedures for identifying the bits represented by the signals transmitted by reader and tag. The maximum-likelihood procedures requires a precise knowledge of the beginning and ending of each bit period which is accomplished by a bit-timing clock signal which originates in a reader and is communicated by the reader to each tag with which it communicates. Both the reader and the tag utilize this common bit-timing clock signal for timing their bit transmissions.